1. Technical Field
The present invention relates in general to data transmission and in particular to extending the distance over which data may be transmitted on a transmission line which inherently distorts the transmission. Still more particularly, the present invention relates to in-situ dynamic selection of pre-emphasis, also called equalization, coefficients to apply to a signal to pre-compensate the waveform for attenuation through a transmission cable.
2. Description of the Related Art
Digital data are typically transmitted over various metallic transmission media, including coaxial cables, in the form of a series of square waves or pulses. Transmission of digital data therefore requires the use of a low-pass channel or cable with a bandwidth large enough to accommodate the essential frequency content of the data stream. High speed digital signals transmitted over cables or other transmission media may be severely distorted because the cable is not ideal and is dispersive in that each received pulse is affected by adjacent pulses, causing a phenomena called intersymbol interference. Intersymbol interference is a major source of bit errors in the reconstructed data stream at the receiver. To correct for intersymbol interference the pulse shape of the signal can be controlled in the overall system. There is a limit, however, to the data transmission rate and/or the distance over which signals may be transmitted because of intersymbol interference.
The data transmission rate for an information bearing signal on a transmission mission line is limited by data jitter, or the tendency of transitions to occur at different points during the respective signal periods. Data jitter arises as a result of two phenomenon: transmission line attenuation typically increases with frequency; and data transitions in adjacent signal periods appear to the transmission line as a signal with a different set of frequencies than transitions which are separated by several signal periods; this is the phenomena that causes intersymbol interference. For example, transmission of the pattern 10101010 will be attenuated differently than transmission of 10011001. The higher attenuation causes signals with irregular transitions to be more severely distorted, with transitions occurring at different points during the signal period than in signals with regular transitions. This limits the pulse width, and thus the data rate, at which data may be accurately transmitted and received.
The distance over which an information bearing signal may be transmitted on a transmission line is primarily limited by the phase distortion introduced by the transmission line. Progressive alteration of the pulse shape from the predetermined pulse shape for which a receiver is designed to operate may lead to transmission errors and limit the maximum range of the transmission.
Several prior art approaches attempt to alleviate the problems arising from distortion. One approach, known as receiver equalization, involves processing the received signal to make it more nearly correspond to the predetermined pulse shape. However, receiver equalization is often complicated by overlap in adjacent pulses in high speed transmission systems.
A second prior art approach, referred to as predistortion or transmitter equalization, employs a separate mechanism at the output of the transmitter to alter the shape of the pulse introduced onto the transmission line. The predistortion is intended to be the inverse of the distortion resulting from inherent cable losses. This technique is suitable when the length and material of the transmission line is known so that the distortion can be estimated and a fixed predistortion factor can be preprogrammed. Another predistortion technique applies voltage to the output signal based on whether the previous bit is identical or different than a bit. Because cable distortion affects all bits in the data stream and is not limited to specific bits, this technique is limited in how closely the predistortion can be made to match the inverse of the cable distortion.
Transmitter or receiver equalizers are the most commonly used technique for dealing with cable attenuation. Passive external components are used to form a filter circuit which has a response approximating the inverse of the cable response, with the xe2x80x9cgainxe2x80x9d of the filter set to equal the loss of the cable.
Another prior art approach to countering transmission line distortion, which may be used in conjunction with receiver or transmitter equalization, involves special encoding of the data to be transmitted. However, predistortion techniques for such signals may be unsuitable for transmitting unencoded or non-return-to-zero (NRZ) data.
It is therefore an object of the present invention to provide improved data transmission and to provide a method of dynamically compensating signals prior to transmission of the signals over a transmission line.
The foregoing object is achieved by a method and apparatus which dynamically selects appropriate pre-emphasis coefficients based on an in-situ determination of the network attenuation characteristics and then to apply a pre-emphasis coefficient to each data bit of the data transmission as the bits are transmitted based on the history of the data transitions. From a group of several pre-emphasis coefficients, one is selected from one of several described techniques, e.g., measuring the length of the cable, measuring the error rate of returned/received signals, and/or measuring the slope of the returned/received signals at two different voltages.
In one embodiment of the invention, a method of digital signal transmission in a conductive network is disclosed, the method comprising the steps of transmitting a series of digital signal pulses from a driver through a transmission medium to a receiver in said conductive network, determining at least one of a plurality of attenuation characteristics of the conductive network between the driver and the receiver, dynamically providing a pre-emphasis coefficient to each bit of the digital signal pulses as it is being transmitted, wherein the pre-emphasis coefficient is selected on the basis of one of the determined attenuation characteristic.
One attenuation characteristic that can be determined is the length of the transmission medium, usually a coaxial cable, connecting the driver and the receiver across which the series of digital signal pulses are transmitted. Then based on the length, one of a group of pre-emphasis coefficient registers is selected within which a plurality of pre-emphasis coefficients are stored. Then one of a plurality of pre-emphasis coefficients stored within the selected register is selected as determined by a history of at least three of said digital signal pulses. Other attenuation characteristics that can be measured to determine an appropriate pre-emphasis coefficient are the transition frequency of the digital signal pulses, the gauge of the cable, temperature of said network, or humidity of said network.
The method may further be described as transmitting a random signal packet of the digital signal pulses through the transmission medium connecting the driver and the receiver; receiving the transmitted random signal packet either reflected back from or retransmitted from the receiver; incrementing and selecting one of a plurality of pre-emphasis coefficient registers and applying one of a plurality of pre-emphasis coefficients in the selected pre-emphasis coefficient registers to the random signal packet until there are no errors in the received random signal packet. Dynamically providing a pre-emphasis coefficient to each bit of the random signal packet can be accomplished by selecting a pre-emphasis coefficient based on the history of at least three of the bits in the random signal packet.
The invention may also be embodied as a method in which the step of dynamically providing a pre-emphasis coefficient further comprises: transmitting a first test packet of known frequency through said transmission medium connecting the driver and the receiver; receiving the transmitted first test packet either reflected back from or retransmitted from the receiver; determining if the received first test packet has no errors; setting a plurality of pre-emphasis coefficients in a pre-emphasis coefficient register to zero; transmitting a n test packet of n frequency higher than the known frequency through the transmission cable; receiving the transmitted n test packet either reflected back from or retransmitted from the receiver. If the received n test packet has errors, the pre-emphasis coefficient register is incremented and the n test packet is transmitted again and the pre-emphasis coefficient register is incremented again until the received n test packet has no errors. Then a n+1 test packet of a n+1 frequency higher than said n frequency is transmitted through the transmission cable and the n+1 test packet is either reflected back or retransmitted from the receiver. If the received n+1 test packet has errors, the pre-emphasis coefficient register is incremented and the n+1 test packet is transmitted again and the pre-emphasis coefficient register is incremented again until the received n+1 test packet has no errors. When both the n+1 and n returned test packets have no errors, the last incremented pre-emphasis coefficient register is selected. And again, a pre-emphasis coefficient is applied in the driver to each bit of the digital signal pulses as determined by a history of at least three of said bits of said signal packet.
The step of determining at least one of a plurality of attenuation characteristics may further comprise transmitting a test pulse through the cable, providing an first input threshold voltage to the receiver, retransmitting the test pulse through the cable, and providing a second input threshold voltage to the receiver. By measuring the slope of the rise/fall edge of the test pulse between the first and second input threshold voltages, one of a plurality of pre-emphasis coefficient registers having a plurality of the pre-emphasis coefficients to provided to series of digital signal pulses can be dynamically selected. A pre-emphasis coefficient is applied in-situ in the driver to each bit of the series of digital signal pulses as determined by a history of at least three bits of the signal packet.
The invention may also be embodied in an apparatus for digital signal transmission in a conductive network, comprising means for transmitting a series of digital signal pulses from a driver to a receiver; means for measuring one of a plurality of attenuation characteristics of the transmitting means; means for selecting one of a plurality of pre-emphasis coefficient registers, each register corresponding to a range of measurement values of one of the attenuation characteristics; means to select one of a plurality of pre-emphasis coefficients stored in the selected pre-emphasis coefficient registers based on the history of at least three bits of the series of digital signal pulses; and means to dynamically apply the selected pre-emphasis coefficient to each bit of the series of digital signal pulses.
The invention may also be described as an apparatus for digital signal transmission in a conductive network, comprising a driver from which to drive a series of digital signal pulses; a receiver to receive the series of digital signal pulses; a transmission medium connecting the driver and the receiver; a shift register to store at least three sequential bits of the series of digital signal pulses; a plurality of pre-emphasis coefficient registers connected to the shift register and to the driver, each of the registers having a plurality of pre-emphasis coefficients, one pre-emphasis coefficient to be applied to a particular bit of the digital signal pulses as determined by a history of at least three sequential bits stored in the shift register as the digital signal pulses are transmitted onto the transmission medium from the driver; a selector connected to the receiver and the pre-emphasis registers, the selector being enabled to select one of the pre-emphasis coefficient registers as determined by attenuation characteristics of said transmission medium.
The above as well as additional objects, features, and advantages of the present invention will become apparent in the following detailed written description.